This is true as long as the compiler actually can de-virtualize the method, that is, that it can prove that the objects it's handling at a particular call site

This is true as long as the compiler actually can de-virtualize the method, that is, that it can prove that the objects it's handling at a particular call site do not override it. Quite often this is not the case, because of the pattern when the programmer is expected to override / implement an abstract method to plug in the desired functionality. The lookup can be fast if there are few classes involved, and their vtables get cached. JITs can do many fascinating optimizations based on profiling the actual code. They must always be on guard though for a case when their profiling-based conclusions fail to hold with some new data, and they have to de-optimize. This being on guard also has its cost.